1. Field of the Invention
This invention relates to improvements in the thermal barrier properties of silicone resin/glass fiber composites. More particularly, it pertains to the use of a thermal treatment of composites comprising a polysiloxane (silicone resin) matrix with a glass or quartz fiber reinforcement embedded in such matrix in order to effect a chemical change in the structure of the composite from silicone resin into porous silicone dioxide, the latter having improved thermal barrier properties.
2. Description of the Related Art
Missile nosecones and other missile components have recently been manufactured as composite structures that consist of an outer thermal protection layer of a silicone resin/glass fiber material surrounding an inner structural layer of bismaleimide resin/graphite fiber. As the missile approaches high speeds, the surface of the composite structure can reach temperatures up to 1,200xc2x0 C. for brief (less than 2 minutes) periods of time. These temperatures are well above the temperatures under which bismaleimide resin would undergo substantial degradation and destruction, even after short periods of exposure. In some applications other than in the field of space technologies, there exist polymeric surfaces subject to brief exposure to very high temperatures as well.
The purpose of the silicone resin-based layer is to act as a thermal barrier such that the underlying structural layers hopefully experience temperatures only below that at which degradation occurs. This thermal barrier is achieved when the precursor silicone resin is chemically changed to form a relatively thick layer of a porous silicone dioxide.
There is a need to enhance further the thermal protection properties of polysiloxane based composites. As will be seen, the creation of a porous matrix structure in general and of a porous silicone dioxide in particular is helpful in this regard. There have been efforts, however, to improve the susceptibility of this type of composite to erosion, mechanical damage (like impact) and chemical attack through surface modifications.
In particular, U.S. Pat. No. 5,824,404 teaches that oxygen plasma or quartz lamp treatment of an uncoated silicone resin composite can cause the conversion of the polysiloxane groups into silica in the first few micrometers just beneath the surface. This so formed silica is believed to improve the mechanical properties of the composite surface. But the prior art does not teach a method for improving thermal protection of the composite surface via such polysiloxane-silica transformation. The silicone resin composite layer itself provides only a primary, and insufficient, barrier against high temperatures.
While thermal protection is certainly known in the prior art, there remains a need for even better thermal protection technology. The method proposed herein provides such improved thermal protection.
The present invention is directed to a non-contact method for treating a filled silicone resin composite structure such that the thermal protection properties are significantly improved because the thermal flux through the composite layer is significantly reduced. Therefore, any material which lies underneath a protective layer of the filled silicone resin composite will receive superior thermal protection. Such protection is especially important when a material to be so protected is an organic polymer material. As it is well known, organic polymeric materials, with rare exceptions, are generally thermally unstable and degrade and decompose when subjected to elevated temperatures over 300xc2x0 C.
In particular, a bismaleimide resin filled with a graphite fiber is a common material used to manufacture an inner layer of missile nosecones. The heat flux to the such inner bismaleimide resin/graphite fiber layer is reduced by the present invention thus preventing thermal decomposition and outgassing of this inner layer material. Reduced outgassing is of critical importance in the case of the nosecone since debris sensitive tracking optics are typically enclosed within this type of structure.
To effect the improved thermal barrier property in the material, the composite surface is first coated with a thin layer of a highly optically absorptive material, for instance dispersed graphite. The composite surface is then exposed to a periodic fluence of optical radiation, such as that produced by a bank of pulsed quartz lamps. The absorbed light imparts heat to the near surface region of the composite causing chemical reactions to occur in the polysiloxane. One consequence of these chemical reactions is the formation of a porous matrix within the resin. The increased porosity acts to decrease the thermal conductivity, and hence the thermal flux capability of the composite structure.
The use of an optically absorptive coating, such as dispersed graphite, in combination with the quartz lamp treatment to produce a porous silica matrix in a silicone resin/glass fiber composite is necessary because without the graphite or similar coating the composite would not absorb the amount of light radiation necessary to cause the formation of the silica matrix.